Circuitized substrates, such as chip scale packages, ball grid array substrates, test carriers, multi chip modules, and printed wiring boards, often include patterns of conductors. In addition, conductive vias can be formed to electrically connect the conductors to contacts, or other patterns of conductors, located on different surfaces or internal conductive planes of the circuitized substrate.
The two approaches in the art used in mass production for forming conductors and associated connections to contacts or vias are additive circuitization using pattern plating, and subtractive circuitization following full panel plating. Typically both approaches start with a multilayer composite board or substrate that has been laminated with an external metal foil commoning layer, and which has been drilled with blind vias or through holes to make subsequent connections to internal wiring. The external metal foil, usually copper, may be thinned by chemical or mechanical means to facilitate further processing. In the typical additive circuitization process the conductor pattern is then defined by patterning a photoresist, and formed by electroplating metal into the defined pattern and drilled vias not covered by the resist. After plating, the photoresist is stripped and the original thin metal commoning layer is etched away leaving a pattern of conductors and plated vias/through holes. In the typical subtractive process, the first step after the multilayer composite board has been laminated with an external metal foil commoning layer and drilled is to blanket plate all surfaces, including drilled vias, to a final conductor thickness. The conductor pattern is then formed by patterning a photoresist on metal features to remain. All unwanted metal is removed by a chemical etching leaving a pattern of conductors and plated vias/through holes.
The additive approach to circuitization is generally capable of producing well shaped conductors with fine spacing, since the conductors are built up into channels predefined by resist. The shape and density of the conductors is limited by the ability to define channels in photoresist. However, additive methods have many challenges including uniformity of plating across the panel and inside the plated through holes, adhesion of the resist through processing steps, and problems associated with removing the thin metal commoning layer after the resist is stripped. These challenges only increase as boards become thicker and more complex. Furthermore, additive circuitization processes that use electroless plating to avoid the need for a commoning layer are very expensive, and the electroless plating baths tend to have unstable characteristics requiring close monitoring. The subtractive circuitization approach is inherently more simple, with less process steps, and is less costly. Since there is no commoning layer to remove after the conductors are formed as with additive circuitization, all problems associated with the commoning layer etch process step are avoided. In addition, very uniform plating thickness is obtained across the panel and inside plated through holes independent of board thickness. A main disadvantage of the typical subtractive process is that it is more difficult to produce substantially rectangularly shaped surface conductors of dense spacing since the process is limited by the ability to etch away surface metal, which will not normally result in the same sharp edge definition that is possible by a photopattern in resist. This disadvantage becomes more pronounced as boards become thicker and features become more dense because the process parameters required to plate inside the high aspect ratio drilled vias of thick boards will result in thicker surface plating, which in turn further limits the ability to produce dense and rectangular shaped conductors.
As circuitized substrates become denser, thicker and more complex, it is increasingly more difficult, and in many cases impossible, to use conventional processes to form the conductors. In particular, the required size, spacing and shape of the conductors most often cannot be achieved by using conventional processes, especially solely with a subtractive circuitization process.
FIG. 1 shows a much-enlarged sectional view, in elevation of a known circuitized substrate 10. The circuitized substrate 10 includes a substrate 12 having a substantially planar upper surface 14 and a plurality of conductors 16 positioned on the substantially planar upper surface of the substrate. A photoimageable photopatterned dielectric material 18 is positioned on an upper surface 20 of plurality of conductors 16.
Plurality of conductors 16 are formed using solely the conventional subtractive circuitization process described above. A conductive layer is blanket deposited on substrate 12, photopatterned with photoimageable dielectric material to expose portions of the conductive layer and then chemically etched to form plurality of conductors 16. The conductive layer includes a side wall 24 therein defining an opening 26. Chemical etching action, being substantially uniform on the exposed portions of the conductive layer, shapes side wall 24 in a curved concave manner and can form undercut regions 28, especially when the thickness of the conductive layer is greater than about 8 microns. The resultant shape in cross-section of conductors 16 is that of a half hourglass. In general, this half hourglass shape has poorer electrical performance characteristics and lower current carrying capability than substantially rectangular cross-sectional shaped conductors of the same height, width, and spacing. Furthermore, the half hourglass shape clearly limits the conductor density (number of conductors per unit area) because conductors of such shape cannot be placed as closely together as rectangular shaped conductors without creating yield (potential shorting), reliability, and electrical concerns. Conductors 16 can be acceptable when electrical performance is not important, that is, when tight spacing between the center to center dimension of the conductors is not a requirement, and when there is no need for features, such as vias, to be located between conductors. When one or all of these factors is desired, half hourglass shaped conductors are undesirable. Tight spacing between the center to center dimension of plurality of conductors 16 is difficult to achieve by chemical etching without the bases 30 of the plurality of conductors touching one another or being substantially close to touching one another creating a potential short or cross-talk between adjacent conductors.
In the industry today, these problems can be addressed by specifying the conductive layer and the resultant conductors to have a thickness of less than about 8 microns. The undercutting action of chemical etching on a conductor layer having a thickness of less than about 8 microns is of short duration with less pronounced undercutting. However, conductors having a thickness of less than about 8 microns still can have poor electrical characteristics and do have lower current carrying capability. Moreover, about 8 microns of surface copper is not a realistic limit with thick boards that include high aspect through holes that need complete plating throughout, as discussed above. When the thickness of conductors greater than about 8 microns is required in combination with tightly spaced fine lines, sufficiently more etching is required to increase the spacing between the base of the conductors. More etching increases undercutting of the conductors, makes the half hourglass shape more pronounced, and thins the distance between sidewalls of an individual conductor even further resulting in conductors having even poorer electrical performance. FIG. 2 illustrates circuitized substrate 10′ after further chemical etching of circuitized substrate 10 of FIG. 1 to increase spacing between bases 30′ and conductors 16′. Circuitized substrate 10′ has increased spacing between bases 30′ of conductors 16′, however further undesirable undercutting 28′ and an even more pronounced half hourglass shaped conductors exist.
The processes illustrated in FIGS. 1 and 2 are performed uniformly across an entire substrate or panel, and therefore affect every conductor and conductor sidewall in the same way. However, in circuit design for a circuit board or circuit substrate, the total length of conductor involved in areas of tight spacing is normally a small fraction of the total conductor length. For example, tight spacing may be required only on some conductor sidewalls within a fine pitch ball grid array (BGA) site of a printed wiring board, or on the inside sidewalls of two adjacent coupled-pair conductors, but not required on the majority of surface conductors. The process resulting in the structure illustrated in FIG. 1 would be adequate for the vast majority of conductor sidewalls, even though not acceptable in the limited areas of tight spacing. Similarly, the additional etch process, necessary to product tight spacing on only a portion of a circuitized substrate, resulting in the structure illustrated in FIG. 2 would also adversely affect all conductor sidewalls, even where tight spacing was not required. The ideal solution is a process that can differentiate areas of tight spacings from those which can be processed with conventional subtractive circuitization methods.
Accordingly, there is a need in the art for improved processes for fabricating tightly spaced finer patterns of conductors to make a circuitized substrate by utilizing a partial subtractive etching process in the area of the circuitized substrate, where these finer patterns of conductors are desired, which overcomes the disadvantages of the known method and structure.